The Earth is threatened by a powerful creature who destroyed 70% of the Moon with its power rendering into the shape of a crescent moon forever. The creature claims that within a year Earth will also be destroyed by him but he offers mankind a chance to avert this fate. In class 3-E at Kunugigaoka Junior High School he starts working as a homeroom teacher where he teaches his students not only regular subjects but the ways of assassination. The Japanese government promises a reward of ¥10 billion (i.e. 100 million USD) to whoever among the students succeeds to kill their teacher whom they have named "Korosensei" . However this has proven to be an almost impossible task as he has several inhuman abilities at his disposal including the capacity of moving at Mach 20.